These are basically some initial changes for T54115.
* Add sidebar region for navigation tabs
* Rename "User Preferences" to "Settings" in the UI
* Rename "Editing" section to "General"
* Add placeholder sections for workspaces
* Initial grouping/categorizing of settings sections
|{F2386316}|{F2386318}|
Although most changes are trivial, some affect compatibility, so rather have another pair
of eyes check over them. I'd like to commit this to blender2.8 and continue
work then (check on final sectioning, discuss final layout & contents of each
section, add search button, etc).
I'm not sure about how I made `uiLayout.prop` able to use sections for expanded enums.
NOTE: All changes here are tentative, some are even just experiments. Nothing is set in stone.